Benzene is an important industrial chemical (greater than 2 billion pounds produced annually in U.S.) And component of gasoline. It is also an established human leukemogen. The shape of the dose-response curve for benzene-induced leukemia at exposures below 10 ppm is highly controversial and a key public policy and risk assessment issue involving potentially billions of dollars. Conventional epidemiology and toxicology studies are unlikely to resolve this controversy. Work performed to date on this grant has led to the development of a mechanistically- relevant biomarker that should be sufficiently sensitive to shed light on the dose-response curve in the low dose region. This biomarker is the simultaneous detection of leukemia-specific aberrations in chromosomes 5, 7, 8 and 21 by fluorescence in situ hybridization in metaphase cells. We propose to examine this and other biomarkers in the blood of 300 workers exposed to a wide range of benzene concentrations (0.5 to greater than 10 ppm) and 150 unexposed matched controls. The first year will be devoted to the collection of these biological samples with detailed exposure information and sophisticated sample processing such that many new endpoints can be examined. Prior work on this grant has also shown that specific chromosomal translocations related to leukemogenesis can be detected by the reverse transcriptase-polymerase chain reaction (RT-PCR) in benzene- exposed workers. Recent advances in kinetic real-time PCR now make it possible to quantitate accurately the level of various chromosome translocations. We propose to use quantitative real- time PCR to measure levels of the translocations t(8;21), t(9;22), t(11q23) and t(14;18) in the peripheral blood lymphocytes and granulocytes of workers exposed to benzene and unexposed controls. Our studies to date also suggest that individuals differ in their susceptibility to benzene toxicity in relation to their folate status and genetic make-up. We therefore propose to further examine the role folate, vitamin B12, and various genetic polymorphisms play in individual susceptibility to benzene-induced chromosome damage. These studies will contribute to our understanding of the mechanism of benzene-induced leukemia, the risk benzene poses at low doses, and factors associated with susceptibility to this compound.